Adds isNodeAttached helper method.

PR 1 for `onAttach`

This adds a helper method `isNodeAttached`. It lets you know if an element is a child of the document or not. I also updated `triggerDOMRefresh` to take advantage of the new helper.
> Mmm...I disagree. For one, we don't stub any of the other jQuery methods we rely on all throughout Marionette, of which there are many.

yeah I don't think we are really proxying any jquery methods like this anywhere else in the code base. 

> Further, there's no other way to test this other than literally rewriting the jQuery method itself, which we don't do because that's the whole reason that we're using jQuery. Right?

Yes correct, so why test the fact that jquery works? we should test the contract of the method which is that it returns true or false depending on if a  node is attached.

"given a dom node is attached" is the before setup for test 1

"given a dom node is not attached" is the before step for test 2